Weather For Dummies. John D. Cox

       About 1,000 tornadoes

       Approximately ten severe winter storms

       An estimated 2,000 flash floods

       Plus threats most years from tropical storms and hurricanes in the North Atlantic, the Gulf of Mexico, and the Caribbean Sea

      Why all these extreme weather events? It’s like they say about the real estate business: The three most important things are location, location, and location.

As Figure 3-2 illustrates later in this chapter, one region or another of the continental United States is in pathway of a variety of moist and dry and warm and cold air masses coming at it from all directions.

      Winter air masses

      

Continental polar air masses that move down out of the snow-covered regions of northern Canada and Alaska are often in the picture when bitterly cold winter weather visits the United States (see Figure 3-2). Winds of this frigid, dry air blows over the northern plains and through the Midwest and the Northeast and occasionally reaches as far south as Texas and Florida. Out west, the Rockies, Sierra Nevada, and Cascade Mountains usually protect the Pacific Coast. As these air masses move southward, their extreme cold tends to modify, protecting the southern states from the most extreme cold. The barrier of the Appalachians sometimes protects the cities of the East Coast from the worst of it.

Maritime polar air masses regularly sweep over the West Coast from out of the northern Pacific Ocean, bringing cool, moist air that dumps heavy snow in the western mountains. By the time this air crosses east of the Rockies, it is relatively dry. It warms as it blows down the eastern slopes of the mountains and sometimes brings fair weather and warming temperatures to the plains. Maritime polar air also originates in the North Atlantic and occasionally sweeps southwestward into New England and the mid-Atlantic states, bringing rain and snow.

      FIGURE 3-2: The map shows the different air masses that affect weather in the continental United States and helps explain why the nation gets so much dramatic weather.

      LAKE-EFFECT SNOWS

      In late fall and early winter, people who live along the southern and eastern shores of the Great Lakes are very familiar with the effects of big expanses of water on the air. The continental polar air mass that brings cold, clear winter days to the Midwest has a very different look to it by the time it crosses the Great Lakes.

      Even after — or if — the lakes freeze over, still they affect the amount of snowfall in the region. Four main weather-generating processes are at work when the cold, dry Canadian air flows over the lakes and the regions along the southern and eastern shorelines.

       The dry air absorbs moisture from the lake, which fuels the snowfall and gives the air more latent heat energy that is released as it condenses into a cloud.

       Flowing over the warmer lake water, the temperature of the cold air rises, and as it warms, the air lifts higher into the sky and becomes more unstable.

       After blowing over the smooth surface of the lake, the air plows into the uneven surface of the land, slows down in the friction, and piles up, rising farther into the sky.

       And the cold air gets another lift from the hills and higher terrain it encounters above the far shores.

      In the summer, the Great Lakes have a very different effect on the weather of the region. Because the water of the lakes is often cooler than the surrounding air this time of year, they can dampen thunderstorm activity.

Maritime tropical air masses from the Pacific sometimes sweep northeastward over California, and when they do, they can bring heavy rains and flooding. Similarly, maritime tropical air from out of the Gulf of Mexico and Caribbean Sea can fuel flooding storms as it moves up the Mississippi and Ohio valleys.

      Summer air masses

      The continental polar air that brought frigid misery during winter often has a welcome cooling effect over much of the United States in summer. Fair weather generally prevails over the Northern Plains and Midwest when continental polar air is around, occasionally reaching even the Gulf of Mexico with pleasant dry air.

      Maritime tropical air masses from the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico spend a lot of time over the eastern two-thirds of the United States during summer, especially the Southeast, bringing warm temperatures and high humidity.

      Continental tropical air forms over the big desert regions of northern Mexico and the U.S. Southwest in summer, keeping things hot and dry.

News from the Fronts

      During the most powerful storms of winter and summer you will probably find two air masses of very different qualities — different temperatures and humidities — crashing into one another. When upper winds and other conditions are right, this is where you will see a powerful mid-latitude cyclone with well-defined fronts that bring rain or snow over big sections of the country. In spring and summer, along these fronts can be long lines of severe thunderstorms that can form tornadoes when conditions are right — or wrong!

      It might look like a rainy or snowy mess from down below, but a front, the transition zone or line between two distinct air masses, has a certain shape or structure to it, depending on what’s going on.

      Cold fronts

When a cold front moves into space occupied by warmer air, it forces the warmer air to rise up and over the leading cold wedge. This is because cold air is denser, or heavier, than warm air and sort of bullies it out of the way. As the warm air rises, it mixes with the cold air and condenses into big clouds. Figure 3-3 shows what happens with the passage of a typical cold front.

      FIGURE 3-3: These diagrams illustrate what usually happens when cold fronts and warm fronts come along.

Often, these cold fronts are carried along by westerly winds, which tear away the tops of these clouds and carry them out far in advance of the approaching front. The first sign of an advancing cold front can be the appearance of these high clouds, followed quickly by a thickening sky. And then a line of heavy thundershowers develops as the front passes, followed by steadier and gradually lighter rain. A typical cold front moves about 30 miles an hour, often traveling to the south and east.

      Warm fronts

      When a warm front moves into the space occupied by a retreating cold air mass, it develops a completely different precipitation pattern than an advancing cold front would. While a cold front digs in underneath a warm air mass, because its air is denser, the lighter air of a warm front rides up above the cold air it is replacing. In this rising warm air, clouds form far out ahead of the advancing warm front’s boundary. Hundreds of miles ahead of the surface edge of a warm front, the cloud cover slowly thickens, finally bringing rain or snow. All this happens long before the surface edge of the warm front finally passes. When that happens, the sky is usually clear. A typical warm front can travel about 10 to 20 miles per hour, most often moving north and east.